The present invention relates to apparatus for use with polymerically insulated electric cables and more specifically relates to a knife used to strip the protective jacket from a polymerically insulated cable.
Polymerically insulated cables are now the predominant cable construction type for both high voltage and low voltage power transmission. In low voltage applications, a tough polymeric jacket layer is extruded concentrically about the central conductor. The polymeric jacket serves to electrically insulate and environmentally protect the conductor.
In high voltage applications, one type of polymerically insulated high voltage cable, typically used by electric utilities, is commonly referred to as jacketed cable. The jacketed cable has a number of spirally wound metallic wires which form a centralized conductor. Concentrically disposed about the conductor is an insulation layer which electrically insulates the high voltage conductor from the surrounding environment. The insulation layer is surrounded by a concentric grounded non-metallic shield. Surrounding the shield and electrically connected to the shield is a series of concentric neutral conductors which are typically spaced about the circumference of the shield and spirally wound along the length of the cable. These concentric neutral wires function to ensure the non-metallic shield is grounded along the entire length of the cable and to conduct any leakage current away from breaks in the insulation layer. To protect the non-metallic shield and insulation layer from the intrusion of water or physical objects, the concentric neutral wires are surrounded by a tough resilient polymeric jacket. In the manufacture of high voltage polymeric cable, the protective jacket is extruded about the concentric neutral cables and non-metallic shield very tightly to remove any possibility of entrapped air bubbles.
The insulation layer is designed to adequately prevent any leakage of current from the high voltage conductor to the grounded exterior The insulating layer's thickness and material selection is predicated on the non-metallic shield circumferentially surrounding the insulating layer in a smooth concentric distribution. If there is any removal of or breaks in the non-metallic shield, a high voltage stress gradient can develop which may cause a steady degradation of the insulation layer until sufficient current can penetrate the layer thereby causing a cable failure. These cable failures are very costly not only from the expense of repairing the cable but also due to the deprivation of electric power to the location to which the cable was supplying power.
It is well known that these type of cable failure where there has been a break in the non-metallic shield typically occurs at points where the cable has been either spliced or terminated for connection to an electrical apparatus. One reason for a cable failure is faulty preparation of the cable prior to the splicing or terminating of the cable. Proper preparation of the cable requires the outer jacket to be carefully stripped back to prevent the destruction or disturbance of the non-metallic shield.
To prepare high and low voltage cable, a significant circumferential portion of the protective jacket must be removed along the length of the cable by slicing. Then the remaining jacket along the sliced portion is grasped and circumferentially peeled from the cable. If an insufficient portion of cable is removed during the slicing step, the peeling step may be impossible thus necessitating additional slicing. The two step stripping operation is often performed in environments such as darkened manholes or far above the ground along insulated power lines which make the stripping operation even more difficult.
The principle device for stripping the protective jacket from a high or low voltage cable is the common pocket knife. This device is favored because it is easily transportable and can be sharpened with ordinary sharpening stones.
The pocket knife, however, has significant drawbacks in its use as a cable stripping or preparation tool. The principle drawback is that the straight edge of the blade of the knife is ill-suited for stripping a jacketed cable having a circular periphery. During the slicing operation, the straight blade edge will typically not remove a large enough circumferential portion of the protective jacket to allow the remaining circumferential portion to be peeled away. Therefore, extra slicing steps along the cable are needed to remove additional amounts of jacket.
Another drawback is that the knife has a handle on only one side of the blade. Therefore, as the worker draws the blade longitudinally along the length of the cable, the knife will frequently pivot about a vertical axis and cause the blade to bind in the insulation or slide off the cable. Upon the knife sliding off the cable, the blade may strike a worker and cause injury.
A related drawback is the knife's singular handle may not allow the cutting angle of the blade relative to the neutral conductors to be adequately controlled as the blade is drawn along the cable. The lack of control of the cutting angle may cause the blade to cut into the neutral conductors and damage the cable or skip radially outward from the neutral conductors thereby leaving a residue of jacket which may hinder or prevent the peeling removal of the remaining circumferential portion of jacket. Also, should the blade slice through the neutral conductors, the ends of the conductors may form sharp points which will damage the heat shrink tubing generally employed to environmentally seal the completed splice or termination.
A further drawback is that the knife has a pointed tip and as the blade is drawn along the high voltage cable, the tip may dig into the non-metallic shield and damage the shielding layer, which as before-mentioned, could cause the cable to eventually fail.
The above listed drawbacks lengthen the time and effort to strip the protective jacket from the cable which may cause worker fatigue.
Thus, there is a need for a device to quickly and easily remove a significant circumferential portion of the exterior protective jacket on jacketed cables and allow removal of the remaining jacket by peeling about the circumference of the cable. There is also a need for a device which is easily transportable and yet can slice away this significant portion without pivoting, making irregular cuts and creating an injury hazard. There is a further need for a device which does not have a sharpened tip which may penetrate and damage the non-metallic shield in high voltage cables.
It is therefore an object of the present invention to provide a device for use with polymerically insulated cable. It is a related object to provide such a device to strip the protective jacket from a polymerically insulated cable which is easily transportable.
It is an additional object of the present invention to provide a device for stripping a jacketed cable having a circular periphery by removing a significant exterior portion of the jacket in a single slice thus eliminating extra slicing steps.
It is a further object of the present invention to provide a device which may be drawn along the cable without pivoting about a vertical axis, thereby causing the device to bind in the jacket or slide off the cable and create a safety hazard.
It is a still further object of the present invention to provide a device in which the cutting angle between the device and the neutral conductors may be easily and accurately controlled to prevent the damaging of the cable or the leaving behind of jacket residue after slicing away a portion of the cable.
It is a still further object of the present invention to provide such a device which does not include a sharpened tip which may damage the non-metallic shield of a high voltage cable.